Many wells are fractured with a fracturing fluid to treat a formation and improve oil and gas production. In a standard fracturing operation, fracturing fluid is pumped down a wellbore with high pressure, causing a formation to fracture around a borehole. The fracturing fluid contains proppant (e.g. sand and/or other particles), which remains in the formation fractures and acts to “prop” open the fractures in the formation to increase hydrocarbon flow into the wellbore. Without proppant, the formation fractures may close, reducing the effectiveness of the fracturing procedure. Sometimes, other unwanted effects may occur. This may include proppant flowing back up the wellbore or an uneven distribution of proppant within the fractures in the formation. The resulting hydrocarbon production from the fractured formation may be less than optimal because of these unwanted effects. An example of a reference for hydraulic fracturing and its evaluation is described in the article “Hydraulic fracture evaluation with multiple radioactive tracers,” by Pemper et al., Geophysics, Vol. 53, No. 10 (October 1998), at 1323-1333, which is incorporated herein by reference.
As a result, it would benefit an operator to know the status of the formation after fracturing. If a formation had been minimally fractured, the operator could fracture the formation again. If it could be determined that the formation was evenly fractured, and that much of the proppant was retained in the formation fractures, then the operator could continue with hydrocarbon production.
Logging tools for measuring formation properties before fracturing are known. These tools have been used in the past to log a formation to detect oil and gas formations adjacent to a wellbore. However, there has not been an ability to measure the azimuthal distribution of proppant in formation fractures.
FIG. 1A shows a deployed exemplary neutron logging system as known in the prior art as a cased hole reservoir evaluation tool. This system is similar to the system disclosed in U.S. Pat. No. 7,999,220, which is incorporated herein by reference in its entirety. Other systems are disclosed in U.S. Pat. Nos. 5,374,823 and 6,376,838, which are also incorporated herein by reference.
For the system of FIG. 1A, neutron logging tool 10 is disposed within a borehole 33 penetrating earth formation 40. The borehole 33 may be cased with casing 35, and the casing-borehole annulus may be filled with a grouting material such as cement. Alternatively, the borehole 33 may be an uncased open hole.
Subsection 11 houses an array of detector assemblies 100 as well as a neutron generator 102. More specifically, there are four detector assemblies 100, each comprising a LaBr3 detector crystal and digital spectrometer for filtering and pulse inspection. These detectors are referred to as the proximal detector 104, the near detector 106, the far detector 110, and the long detector 112. The detectors are disposed at increasing longitudinal (or axial or vertical) distances from the neutron generator 102. Between the near detector 106 and far detector 110 is a fast neutron detector 108 that measures the fast neutron output flux and pulse shape of the neutron generator 102.
Subsection 11 is connected to instrument subsection 24. Instrument subsection 24 houses control circuits and power circuits to operate and control the elements of subsection 11. Additional elements of neutron logging tool 10 include telemetry subsection 26 and connector 28. Neutron logging tool 10 is connected by wireline logging cable 30 to above-surface elements such as draw works 34 and surface equipment 36.
Another multi-detector neutron logging tool 10, known in the prior art as a pulsed neutron decay tool, is shown in FIG. 1B. Additional examples of different neutron logging tools 10, in addition to the cased reservoir evaluation tool (CRE) in FIG. 1A and the pulsed neutron decay tool (PND) in FIG. 1B, are the dual neutron tool (MDN), and the compensated neutron tools (CNT-S and CNT-V), all of which are available from Weatherford International Ltd.
The prior art neutron logging tools, such as tool 10 in FIGS. 1A-1B, are not able to give azimuthal logging information. Rather, the two or more detector assemblies 100 are spaced apart longitudinally along the body of the neutron logging tool 10 a short distance from the neutron source 102, and the detector assemblies 100 are vertically in line with each other along a central axis of the tool. Yet, the detector assemblies 100 make their detections of the adjacent wall of the borehole without particular regard to direction or orientation. Instead, the intention of the multiple detector assemblies 100 is to provide different formation and statistical sensitivities during logging operations.
In particular, the effect is that the detector assemblies 100 closest to the neutron generator 102 typically are more sensitive to the borehole 33, and the detector assemblies 100 further from the neutron generator 102 typically are more sensitive to the overall formation 40. The sigma (Σ) capture cross-section of the borehole 33 and formation 40 of the readings may be computed by giving different weights to the near detectors' readings as compared to the far detectors' readings. For example, in a tool with two detectors, 70% weight may be given for the near detector reading and 30% weight for the far detector reading. The neutron logging tool 10 is usually run decentralized to the wellbore with an offset spring, or decentralizer, (not shown) such that the neutron logging tool 10 effectively runs along one wall of the wellbore.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.